This invention relates to a variable displacement rotary pump and more particularly, to a control valve which controls the pump displacement in response to the rate of fluid flowing from the pump.
Most currently available vehicle power steering pumps are of a constant displacement design. They provide an essentially constant output fluid flow rate at varying engine speeds by internally bypassing excess flow to the reservoir. However, the power consumed by such pumps increases substantially with increasing engine speed. Typically, these pumps also include a separate high pressure relief valve for bypassing excess flow to the reservoir in response to blockage of a pressure line. In either case, the resulting high fluid flow rate can cause heating of the hydraulic fluid and pump failure. As a result, coolers may be required to maintain the fluid temperature within acceptable limits. Existing variable displacement axial piston-type pumps include a rotating barrel which carries reciprocating pistons engageable with a tiltable swash plate. The swash plate is usually biased to full stroke by spring or hydraulic means and reduction of stroke is accomplished by overcoming the bias load with a control piston operated by system pressure or some fraction of system pressure. In some cases stroke is reduced mechanically via a lever attached to the swash plate trunion. These pumps typically operate at constant speed and flow except where system pressure exceeds a preset level. When this occurs the control piston moves to reduce stroke and flow to maintain the desired system pressure level. The pump and control described herein are designed to maintain a relatively constant fluid flow rate over a wide range of speed and pressure conditions. In the event system pressure exceeds a predetermined level the flow control circuit is biased and displacement is reduced thereby minimizing heat input to the fluid.